Method and system of reusing walsh codes to increase forward channel capacity

ABSTRACT

Assigning CDMA channel codes in a multiple beam zone sector such that they may be reused within the same sector. In an exemplary embodiment, the method includes configuring directional antennas in a cell sector to provide multiple beam zones in a cell sector and assigning channel codes for use within the beam zones such that each channel code is assigned for use in only one beam zone of any set of three consecutive beam zones in the cell sector. The reassignment is preferable conditioned upon the mobility status of the user. This effectively reduces the inter-user interference by ensuring at least two intervening beam zones are located between zones that are using the same channel codes, and that the second user is not mobile. The channel codes may be Walsh codes or quasi-orthogonal functions (QOFs).

BACKGROUND

A. Field of Invention

The present invention is related to wireless communication systems, andmore particularly, to a method and system of utilizing Walsh codes in asectorized CDMA cellular communication system having multiple beamzones.

B. Description of Related Art

In a typical wireless communication system, an area is dividedgeographically into a number of cell sites, each defined by one or moreradiation patterns created by an emission of radio frequency (RF)electromagnetic (EM) waves from a respective base transceiver station(BTS) antenna. Each cell site is typically further divided into two,three, or more sectors, where the sectors provide radio coverage for aselected area within the cell site. Each sector of the cell typicallyuses dedicated antennas to provide the required coverage.

In CDMA communication systems, each sector uses a unique PN code(commonly referred to as a short PN code offset) to distinguish itselffrom surrounding sectors and cells. Within each sector, channels aredistinguished by yet another code, termed a Walsh code. In an adjacentsector, the Walsh codes may be reused because channel separation isprovided by a different offset of the short PN code for that sector.Thus, the number of available forward channels (BTS to MS) on a givencarrier frequency in a sector is limited by the number of availableWalsh codes. In the ANSI/TIA/EIA-95-B-99 standard entitled “MobileStation-Base Station Compatibility Standard for Wideband Spread SpectrumCellular Systems” (published Feb. 1, 1999), the contents of which areincorporated by reference herein, there are sixty-four available Walshcodes, while in CDMA 2000 series (TIA/EIA IS-2000 Series, Rev. A,published Mar. 1, 2000), one hundred twenty-eight Walsh codes areavailable. Both of the ANSI/TIA/EIA-95-B-99 and the TIA/EIA IS-2000Series, Rev. A, standards are incorporated herein by reference, and areavailable from the Telecommunication Industry Association, 2500 WilsonBoulevard, Suite 300, Arlington, Va. 22201.

The maximum number of forward channels for a single RF carrier istherefore fixed in a given CDMA cell site having a particular sectortopology. To increase system capacity, quasi-orthogonal functions (QOF)may be used to supplement the available Walsh codes. QOFs are derivedfrom Walsh codes, and are not perfectly orthogonal to the set of Walshcodes. Nevertheless, they provide a relatively low level of inter-usercross-correlation interference that makes them suitable for use whenextra capacity is needed. Techniques of generating QOFs are well known,and are based on a mask value and masking function, from which an entireset of QOFs may be generated by masking the original set of Walsh codes.Accordingly, each QOF mask may be used to generate a QOF set.

Oftentimes, however, the capacity of the system is reached before all ofthe Walsh codes have been used due to interference between the CDMAusers in a sector or nearby sectors. To reduce interference, smartantennas may be used to limit the amount of signal power received frominterfering users.

A smart antenna may actually be an array of antenna elements workingtogether to produce a particular radiation pattern. Each antenna in thearray is referred to as an antenna element (or simply an element). Anantenna radiation pattern is also referred to as an antenna-beam or beamzone. A beam width of an antenna is a measure of directivity of anantenna and is usually defined by angles where the radiation patternreduces to one half of its peak value or more commonly referred to as 3db points (i.e., 3 decibel power level). Using sophisticated antennaarrays, a given sector may be divided into directional sub-sectorscovered by one or more beam zones.

The use of smart antennas to reduce interference may allow an increasein system capacity. However, the number of available Walsh codes inconjunction with the QOFs, may still limit a sector's capacity.Consequently, a system that overcomes these limitations is desirable.

SUMMARY

Provided is a method of assigning CDMA channel codes in a multiple beamzone sector such that they may be reused within the same sector. In anexemplary embodiment, a method is provided that includes configuringdirectional antennas in a cell sector to provide multiple beam zones ina cell sector and assigning channel codes for use within the beam zonessuch that each channel code is assigned for use in only one beam zone ofany set of three consecutive beam zones in the cell sector. Thiseffectively reduces the inter-user interference by ensuring at least twointervening beam zones are located between zones that are using the samechannel codes. The channel codes may be Walsh codes or quasi orthogonalfunctions (QOFs). If a given channel code is in use by a first userwithin a sector, then before assigning the same channel code to a seconduser within a different beam zone, the system preferably checks todetermine whether the second user is moving (changing its physicallocation) or fixed. In the event that the second user is fixed, then theparticular channel code may be assigned to the second user. If, however,the second user is moving, the channel code will not be additionallyassigned to the second user, thereby reducing the possibility ofdegraded service when the user moves closer to the beam zone where thechannel code is already in use.

In some embodiments, the channel codes are dynamically assigned to thebeam zones, while in alternative embodiments they are staticallyassigned. In embodiments using dynamic assignment, the method includestracking currently assigned channel codes and channel code availabilityfor each of the two or more beam zones. Various techniques may be usedto determine channel code availability, such as by tracking the channelcode usage in adjacent beam zones. If the sector has numerous beamzones, the method may examine up to four or more adjacent beam zoneswithin the sector to ensure interference is minimized.

Overall system capacity may be reserved by limiting the number of codesthat are assigned to a particular code group, or beam zone. This may bedone by setting a maximum number of channel codes that may be used inany beam zone of the sector. In addition, the maximum number of channelcodes for each beam zone may be dynamically adjusted, based on the timeof day, the number of calls, or other sector and cell-specificstatistics.

In embodiments using static assignment of channel codes, the assignmentsmay nonetheless be periodically modified, such as every few minutes, orhours.

In certain embodiments, code management is performed by statically ordynamically assigning channel codes to certain groups, where each groupis intended for use in predetermined beam zones, where the beam zonesare arranged so that beam zones using the same code group are separatedby at least two intervening beam zones in the cell sector. In otherembodiments, groups are not maintained, but instead the use of codes andtheir availability for use is determined by examining current channelcode assignments within the relevant adjacent beam zones.

These as well as other features and advantages of the present inventionwill become apparent to those of ordinary skill in the art by readingthe following detailed description, with appropriate reference to theaccompanying drawings.

BRIEF DESCRIPTION OF FIGURES

Reference is made to the attached drawings, wherein elements that havethe same reference numeral designations represent like elementsthroughout and wherein:

FIG. 1 is a block diagram illustrating one embodiment of a wirelesscommunication system;

FIG. 2 illustrates one embodiment of a cell site;

FIG. 3 illustrates a cell site radiation pattern having multiple sectorsand beam zones;

FIG. 4 illustrates adaptive beam zones in a single sector;

FIGS. 5A and 5B illustrate alternative methods of assigning channelcodes;

FIG. 6 is a flowchart depicting functional blocks of a method accordingto one embodiment;

FIGS. 7A and 7B are flowcharts depicting functional blocks of a methodaccording to alternative embodiments; and

FIG. 8 is a flowchart of an alternative method of assigned channelcodes.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS A. Wireless CommunicationSystem

In accordance with an exemplary embodiment, a method and system ofassigning Walsh codes for use within a wireless communication system isprovided. Referring to FIG. 1, a block diagram illustrating oneembodiment of a wireless communication system 100 is provided. It shouldbe understood that this and other arrangements described herein are setforth for purposes of example only, and other arrangements and elementscan be used instead and some elements may be omitted altogether.Further, as in most telecommunications applications, those skilled inthe art will appreciate that many of the elements described herein arefunctional entities that may be implemented as hardware, firmware and/orsoftware, and as discrete components or in conjunction with othercomponents, in any suitable combination and location.

By way of example, the wireless communication system 100 is shown toinclude a mobile station (MS) 102 in communication via an air interface104 with a base transceiver station (BTS) 106, which is coupled to abase station controller (BSC) 108. The BSC 108 is also coupled to anetwork 114. Mobile stations such as cellular telephones, personaldigital assistants (PDA), wireless modems, or other mobile nodes may beused in the wireless communication system 100.

BTS 106 includes one or more antennas arranged to produce radiationpatterns defining one or more sectors. Additional BTSs 110 and 112coupled to BSC 108 are also illustrated. Although three BTSs areillustrated within FIG. 1, it will be understood that more or fewer BTSsmay be present within the wireless communication system 100.

BSC 108 is an interface between BTSs 106, 110, and 112 and the network114. BSC 108 also handles radio resource management and radio networkmanagement functions for BTSs 106, 110, and 112.

The network 114 may be any transport network and/or entity used to routesignals to and from the MS 102. For example, network 114 may comprise amobile switching center (MSC), a packet data service node (PDSN), anInternet protocol (IP) network, the public switched telephone network(PSTN), or any other wireless communication transport network. Inaddition, network 114 may allow for connectivity to multiple switchingplatforms, such as a short message service center (SMSC) and an uplinkserver, for example.

Each of the couplings of the wireless communication system 100,excluding the air interface 104, may be interfaces using variousphysical, media access, and data link layer technologies, including suchtechnologies as TDM trunks (e.g., trunk level 1 (T1) line), apacketbased link (e.g, IEEE 802.3, gigabit Ethernet line), or otherconnections.

The wireless communication system 100 may be divided geographically intoa number of cell sites. At the core of each cell site is a BTS, such asBTSs 106, 110, and 112, whose antennas define a radio frequency (RF)radiation pattern. Further, each cell site may be divided into a numberof sectors, each defined respectively by radiation patterns fromdirectional antenna elements of the cell site's BTS. Each sectortypically has a beam width of about 120 degrees. However, sectors canhave any desired beam width.

The radiation pattern of each sector in a cell site produces an airinterface that defines a respective coverage area, such as air interface104. When an MS is positioned within such a coverage area, the airinterface provides a communication path between the MS and the BTS. Andthe MS can then communicate through this path with entities on thewireless communication system 100.

In the wireless communication system 100, the BTSs 106, 110, and 112 maytransmit signals using one or more carrier frequencies. A carrierfrequency (or simply carrier) is a transmitted electromagnetic pulse orwave at a steady frequency of alternation on which information can beimposed by increasing signal strength, varying the frequency, varyingthe wave phase, or other means. This variation is referred to asmodulation.

In a typical CDMA wireless communication network, a subscriber connectsto the base station and the network infrastructure by way of twoseparate wireless channels—one from the BTS to the MS, typicallyreferred to as the forward channel, and one from the MS to the BTS, orthe reverse channel. The forward and reverse channels have differentformats and utilize different physical and link layer signaling. As aresult, the channel characteristics, including their overall capacity interms of the number of channels available may vary between the forwardand reverse channels.

Each channel in a CDMA system is identified by a number of parameters,including the frequency of the RF carrier, and various PN sequences thatare sequentially applied to the data. In particular, the forward channeluses a long PN code sequence to scramble the data, and a short PN codewith a time offset that defines the sector. That is, all forwardchannels in a given sector use the same PN short code with the sameoffset. Other sectors may utilize the same short PN code, but with adifferent offset.

In addition, each forward channel in a sector uses a specific PN Walshcode to identify data for a given user. The orthogonality of the Walshcodes separates the user channels within a given sector. In IS 95, thereare sixty-four Walsh codes—one is used as a pilot, one (or from one toseven) is used for paging, and one is used for sync, leaving as many assixty-one Walsh codes for sixty-one forward channels per sector. IN CDMA2000, 128 Walsh codes are available. In either system, adjacent sectorsmay use the same set of Walsh codes due to the use of different short PNcode offsets in those sectors.

For the reverse channel, the various PN codes are used in a differentmanner. First, each MS uses the Walsh codes as a symbol alphabet,whereby up to six data bits may be combined and represented by a singleWalsh code. The short code is then applied, and is used forsynchronization purposes. Finally, the individual user channels areidentified by the offset of the long code. More particularly, thespecific long code used to identify the MS's channel is made by“masking” the PN long code by a number determined mathematically by thehandset's ESN. With over forty days of 1,228,800 chips/second to choosefrom, there are billions and billions of reverse traffic channelspossible. Of course, reverse access channels are associated with eachpaging channel in the forward direction, which are publicly-defined longcode offsets reserved for reverse-direction public traffic such as calloriginations, registrations, etc. After a BTS recognizes an MS on anaccess channel, its identity is known and the BTS redirects the MS to atraffic channel where it will use its own natural long code.

B. Types of Antennas

In one embodiment, the antennas of the antenna-arrangement 204illustrated in FIG. 2, the antennas of the antenna-arrangement 302illustrated in FIG. 3 are directional adaptive beam-forming antennas. Anadaptive antenna array (AAA) arrangement can adjust to an RF environmentas it changes, and dynamically alter radiation patterns to optimize theperformance of a wireless communication system. The AAA typically uses aphased array of antenna elements to perform beam steering and nullsteering.

For the receive function of the base transceiver station, the adaptiveantenna-arrangement may isolate interfering signals by calculating theirdirections of arrival. Specifically, by adjusting how the signals fromthe elements are combined, the BTS/AAA can adjust the main lobes and thenulls of the radiation pattern to maximize the signal to noise ration ofthe desired signal and minimize the interference from other users. Theadaptive antenna-arrangement can update its beam pattern in real timebased on changes in both the desired and interfering signal locations.

To perform these functions, the AAA requires information about locationsand RF power levels for the MSs in the coverage area. In addition,knowledge of the relevant Walsh codes, PN code offsets, andsynchronization information is used. Together with the direction ofarrival of the interference and the location of the desired user, theAAA calculates the optimal weighting to apply to the individual antennaelements to place the peaks and nulls of the radiation pattern. Theproper weights may be determined, for example, with least-mean-squared(LMS) or normalized LMS (NLMS) algorithms.

In some antennae systems, each beam zone may operate using signalinghaving a timing offset. By examining the timing aspects of the signalreceived from the MS, the antenna system may be able to determine whichbeam zone the MS is receiving. In turn, this provides the system withlocation information. Alternative techniques of determining locationinformation relating to the MS is understood by those of skill in theart. The location information may be analyzed over time to determine amobility status of the MS. That is, changes in the location informationmay be used to determine whether a mobile station is substantiallystationary, which may be the case when there are no changes in locationinformation, or the changes are below a predetermined threshold. Thethreshold may be in terms of angular variance of the locationinformation, which would be indicative of the rate of movement from onebeam zone area to another adjacent beam zone. The threshold may also bedetermined in relation to an expected average call time, such that thethreshold is related to the amount of angular deviation in the positioninformation that indicates that during the time interval of an averagecall, the MS would not move from one sector to another.

On the transmitter side, location information (e.g., based on desiredfixed beam configurations or information from the receive antennaconfiguration, possibly including beam zone information, the elementweights and resulting receive radiation pattern, etc.) may be used toform transmit beams. The beam forming on the forward link decreases theamount of interference received by other MSs in the system, therebyallowing increased power levels to be used for a given MS.

The antennas of the antenna-arrangement 204 and/or the antennas of theantenna-arrangement 402 may also be fixed-beam or switched-beamantenna-arrangements that may provide individual coverage areas oroverlapping coverage areas. A fixed-beam antenna may consistentlyprovide a similar beam that has a substantially unchanging shape andbeam width so that mobile stations within the beam may be provided acommunication medium by the antenna. A switched-beam antenna-arrangementmay divide coverage areas into several smaller coverage areas. Eachportion of a coverage area is provided by a predetermined fixed-beampattern with the greatest sensitivity located in the center of the beamand less sensitivity elsewhere. The switched-beam antenna-arrangementallows one of several predetermined fixed-beam patterns (based onweighted combinations of antenna outputs) with the greatest output powerto be selected to provide a communication interface between an MS and aBTS. The switched-beam antenna-arrangement switches its beam indifferent directions throughout space by changing phase differences ofsignals used to control the antenna-arrangements.

If adaptive or switched-beam antennas are employed, one antenna mayprovide a communication medium for essentially only one MS, because theantennas dynamically direct their beams to provide a changing coveragearea for the MS. As the MS moves, the antennas adjust the shape andwidth of their beams to accommodate the MS. Alternatively, if fixed-beamantennas are employed, multiple mobile stations may concurrently use theantennas.

As another example, dual-band antennas may be used. For example,antennas designed to transmit within the frequency bands of 824-896 MHzand 1850-1990 MHz may be used. Also, dual band antennas such as a2.4/5.8 GHz dual-band antenna designed for Bluetooth/IEEE-802.11afacility applications may be used in particular for an in-buildingwireless communication system for mobile stations that subscribe tomultiple services. Other types of antennas may be used as well.

A commercially developed intelligent antenna technology that is suitablefor use with the methods and apparatus described herein is one providedby Nortel Networks. Nortel's system is referred to as Adaptive AntennaBeam Selection (AABS) technology.

A further alternative antenna technology suitable for use is Lucent's“Bell Labs Layered Space Time” (BLAST) technology as described in U.S.Pat. No. 6,317,466, the contents of which are incorporated herein byreference.

C. Channel Code Assignment

As described above, forward channels in a given sector are determinedbased in part on the Walsh code (short code PN offset and carrierfrequency are also factors). The channel codes may be sixty-four orone-hundred-twenty-eight chip Walsh codes or quasi orthogonal functions(QOFs), as described above and in the IS-95 and CDMA-2000specifications.

In an exemplary embodiment, a method of increasing the number ofavailable forward channels in a sector is provided that includesconfiguring directional antennas in a cell sector to provide multiplebeam zones in a cell sector and assigning channel codes for use withinthe beam zones. The channel codes are assigned such that they may bereused within a single sector. In one embodiment, this is achieved byensuring that a given code is used in only one beam zone of any set ofthree consecutive beam zones in the cell sector. In addition, in onepreferred embodiment, a given channel code will not be assigned to asecond user if the second user is mobile. If the second user isstationary, then a channel code that is already in use within the sector(but in another beam zone) may be re-used. By restricting the re-use ofchannel codes to fixed users, the possibility of interference isreduced.

As shown in FIG. 4, beam zones BZ1-BZ6 may all serve a single sector andprovide overlapping coverage for MSs within the sector's area.Assignment of channel codes is performed whereby codes used within asector, say BZ3, are not used within immediately adjacent zones BZ2 andBZ4. This pattern of reuse provides a buffer region between beam zones.Preferably, codes used in BZ3 are also not used in the next adjacentzones BZ1 or BZ5 to provide a larger buffer region between zones usingthe same channel codes.

Because beam zones typically overlap, the provision of a buffer(intervening beam zones between zones using the same channel code)reduces inter beam zone interference. That is, if a buffer region werenot provided, an MS receiving signals from a first beam zone on a givenchannel code may inadvertently receive signal energy transmitted withthe same channel code from an overlapping adjacent beam zone. Thissignal energy would not be orthogonal, and would not be eliminated bythe channel code despreading operation in the MS CDMA receiver.

In addition, providing a larger buffer (two or more intervening beamzones) between channel code re-use will further decrease the possibilitythat a mobile will receive interfering signals. Specifically, it is notuncommon for three beam zones to overlap. In such a situation, an MS maybe assigned channel codes for the beam zone servicing it as well asadjacent beam zones on either side for use in a hand-off. As an exampleof how a single beam zone buffer may be insufficient, consider the casewhere an MS in BZ3 is assigned a first channel code for use in BZ3, andis also assigned a second channel code to monitor in BZ2 for possibleuse (e.g., for use in a handoff). If the second channel code were alsoactive in BZ4, then the MS would be unable to separate signal energyencoded with the second channel code from BZ1 and BZ4. Thus, it can beseen that in many situations, a two beam zone buffer is preferable toreduce the inter-user interference.

In embodiments where channel code re-use is restricted based on themobility of the user, the beam zone buffers may be decreased. This isparticularly the case if it is determined that both users (the firstuser assigned a given channel code, and the second user that isrequesting a new channel code assignment) are determined to bestationary, or fixed.

In certain embodiments, code management is performed by statically ordynamically assigning channel codes to certain groups. The groups may bemaintained in a simple table format, as shown in FIG. 5A. Of course,this may take many forms when implemented in software at the BSC and/orBTS and/or beam zone antenna system. Regardless of where the assignmentsare managed, each group is intended for use in predetermined beam zones.The groups are arranged so that beam zones using the same code group areseparated by a buffer of at least one, but preferably at least twointervening beam zones in the cell sector. The number of desiredintervening beam zones may vary depending on the extent of overlapbetween the beam zone regions. The number of beam zones and the amountof overlap may be determined in advance based on the cell and sectorlayout, especially if fixed beam antennas are used. In alternativeembodiments, especially when adaptive beam antennas are used, the beamzones may be determined based in part or entirely on geographic regions.The geographic regions may be predetermined areas, with fixedboundaries, or may be adaptable in response to changes in antennaradiation patterns.

The selection of channel codes within the groups may be static ordynamic. In some embodiments, the channel codes are selected forinclusion in a certain group, and the groups are assigned to specificbeam zones. The code and group selections may be made based on thedesired capacity of the beam zones within the sector. Then, channel codeassignment for particular users is performed by examining theavailability of the codes within the assigned group.

For dynamic grouping, the groups are modified or updated periodically,such as every few minutes, or hours. Groups may be modified based onchanges in call traffic loads in specific beam zones. In this way,additional codes may be added to a beam zone to provide additionalcapacity when needed. In such a scenario, the code group managementalgorithm may include provisions to limit the number of codes that maybe assigned to a group so as to ensure a reserve capacity for all beamzones. An additional method of providing changes to system capacityinvolves the use of alternative sets of predetermined code groups. Thatis, a first and second set of code groups for use within the multiplebeam zones may be provided. The sets to be utilized may be used based onthe time of day, thereby automatically altering the system capacity toaccount for anticipated changes in typical call loads (e.g., during highcommuter traffic times, extra capacity is provided to beam zonescovering high-volume roadways, etc.). Care must be taken whentransitioning between code group sets to ensure buffer zones areproperly maintained during the transition.

A preferred method 600 of managing code sets is shown in FIG. 6. At step602 the beam zones are configured to provide coverage to the desiredareas of the sector. At step 604, the possible channel codes are dividedinto groups. Preferably, three groups of codes are provided, but two maybe used, or even four or five may be used, depending on the sectorlayout and desired capacity. The groups need not have the same number ofcodes. In addition, as discussed above, the groups may be formeddynamically, and/or multiple sets of groups may be provided. At step604, the groups assigned for use within the beam zones.

In other embodiments, specific groups of codes are not maintained, butcodes are instead assigned within beam zones based on theiravailability. Channel code availability may be determined by examiningcurrent channel code assignments of channel codes within the relevantadjacent beam zones. If the channel code is not used within the desiredbeam zone or in the buffered regions surrounding the desired beam zone,then the code is available for assignment. Once assigned, the code is nolonger available for further assignments within the beam zone or thebuffer region beam zones.

Alternatively, code assignment and availability may be trackedexplicitly. One example of a data table for use with such a codemanagement method is depicted in FIG. 5B. Note that this table onlydepicts three channel codes for simplicity, and a larger table would beused for all possible channel codes. The channel codes in the table mayrefer to Walsh codes or QOFs. In addition, certain Walsh codes might notbe included in the pool of possible channel codes in the table becausesome codes are dedicated for use as pilot channels, paging channels, andother system channels. In FIG. 5B, channel codes are indicated by CC1,CC2, etc, and are listed across the top row of the table. The beam zonesBZ1 through BZ6 are listed in the first column of the table. For a givenbeam zone, the channel code usage is indicated by a flag bit AS (for“assigned” status), and the channel code availability is indicated by aflag bit AV (for “available” status). By convention, a flag bit isreferred to as “set” when it has a true logic value (typically a binaryone), and a flag bit is referred to as “reset” or “cleared” when thevalue is false (a binary 0). Thus, when the AS flag is set, it indicatesthe code is assigned, and when the AV flag is set, it indicates the codeis available. For BZ1, channel codes 1 and 3 are in use, and channelcode 2 is unavailable. Because channel code 1 is assigned in BZ1, it isnot available for reuse until after the buffer regions of BZ2 and BZ3.Thus, the table indicates availability for BZ4, BZ5, and BZ6. Theavailability of a given channel code may also be conditioned on themobility of the user, as described below.

When a new channel code is needed for assignment to a mobile in aparticular beam zone, the method may select any channel code where theAV flag is set. Once the channel code is selected and assigned, thecorresponding AS bit is set (indicating the code is presently assigned)and the AV bit is cleared. In addition, the AV bit is cleared for thebeam zones in the corresponding buffer regions to ensure the same codeis not later assigned within the buffer beam zones.

When a channel code is released, or unassigned, in a given beam zone,the corresponding AS bit is cleared. In addition, the AV bit is set forthe buffer beam zones, but only after ensuring that the beam zones arenot acting as a buffer for another beam zone. As an example, considerthe case where the system releases channel code 3 in beam zone 1, andthe AS bit for BZ1 is cleared. Prior to setting the AV flags in BZ2 andBZ3, the AS bit for BZ4 must be checked. Upon examination, the AS bitfor BZ4 is determined to be in the set state, indicating it is presentlybeing used in BZ4. Thus, channel code 3 is not available for use in BZ2and BZ3, and the corresponding AV flags must remain cleared.

Thus, depending on the required buffer zones, dynamic assignment ofchannel codes from and available pool of codes may include trackingcurrently assigned channel codes and preferably the channel codeavailability for each of the two or more beam zones. In sectors havingnumerous beam overlapping zones, the method may require examination ofup to four or more adjacent beam zones within the sector to ensureinterference is minimized.

The dynamic assignment from a pool of codes may also impose limitationson the beam zone to reserve system capacity. That is, each beam zone maybe required to have a minimum number of available channel codes (i.e.,one or more), and a given channel code might be restricted fromassignment if it would result in a zone having no available codes.Alternatively, may impose a limitation of a maximum number of channelcodes assigned in a given beam zone.

In other alternative embodiments, the required reserve capacity may alsobe dynamic. For instance, the maximum number of assigned channel codesfor each beam zone (or the minimum available codes) may be dynamicallyadjusted, based on the time of day, the number of calls, or other sectorand cell-specific conditions or statistics.

Preferred methods 700, 706 of channel code management are shown in FIGS.7A and 7B. In FIG. 7A, channel code assignments are tracked by recordingthe assignment in the corresponding beam zone record. In embodimentsutilizing the explicit tracking of availability (which is optional), theavailability flag is cleared in the relevant buffer beam zone records.The relevant buffer beam zones for a given beam zone may be determinedby default, such as by a rule that defines the buffer zones to be one,or preferably two, adjacent beam zones on each side of the given beamzone. Alternatively, an additional field may be added to the table ofFIG. 5B to explicitly identify the required or desired buffer zones foreach beam zone.

Method 706 depicts a method whereby channel codes are released byrecording the change in assignment in the given beam zone record, asshown by step 708. In embodiments where availability is also tracked,the method may include setting the AV bits for the buffer beam zones.The AV bits are set because the buffer zones are no longer needed. Ofcourse, the AV bits are not set as shown in step 712 withoutverification that the channel code is not presently assigned in nearbybeam zones as shown in decision block 710. “Nearby” beam zones are thosezones that would also use the beam zone under consideration as a bufferzone. The remaining buffer zones are checked as indicated by the block714.

In the example of FIG. 5B (having a preferred buffer of two interveningbeam zones), when channel code 1 in BZ1 is released, then for thepurposes of setting the AV bit for BZ2, beam zone BZ4 is considered tobe “nearby”, whereas BZ5 is not. For the purpose of setting the AV bitfor buffer zone BZ3, beam zone BZ5 is considered to be “nearby”.

Thus, if the channel code is in use in a nearby beam zone, the beam zoneunder consideration is acting as a buffer zone for the nearby beam zone,and the method will not set the AV bit.

With respect to FIG. 8, an alternative method 800 of channel code re-usethat factors in a user's mobility will be described. When an MS requestsa channel code assignment, the system may first obtain mobility datacorresponding to the new user, as depicted in step 802. The mobilitydata may be obtained using any of a number of techniques describedabove, as well as alternatives known to those of skill in the art. Ifthe status of the MS is “mobile”, as determined at step 804, then thesystem may restrict the channel code assignment to only those channelcodes that are not already assigned (e.g., channel codes that do nothave any bits set in the AS field of FIG. 5 b), as depicted in step 806.If, however, the test at step 804 indicates that the status of the MS is“fixed”, then channel codes indicating availability in the beam zone maybe assigned to the MS, even if they are already assigned in another beamzone within the sector.

D. Other Examples

Those skilled in the art to which the present invention pertains maymake modifications resulting in other embodiments employing principlesof the present invention without departing from its spirit orcharacteristics. Accordingly, the described embodiments are to beconsidered in all respects only as illustrative, and not restrictive,and the scope of the present invention is, therefore, indicated by theappended claims rather than by the foregoing description. Consequently,while the present invention has been described with reference toparticular embodiments, modifications apparent to those skilled in theart would still fall within the scope of the invention.

We claim:
 1. A method of increasing forward channel capacity in a CDMAcommunication system comprising: configuring directional antennas in acell sector to provide multiple beam zones in the cell sector, whereinthe multiple beam zones include at least a first beam zone, a secondbeam zone, and at least two intervening beam zones between the firstbeam zone and the second beam zone, wherein the at least two interveningbeam zones include a third beam zone that overlaps with at least aportion of the first beam zone, and wherein the at least two interveningbeam zones include a fourth beam zone that overlaps with at least aportion of the second beam zone; assigning orthogonal channel codes of aset of orthogonal channel codes to a plurality of channel code groups,wherein the orthogonal channel codes are useable by mobile stationsoperating within the cell sector, and wherein the plurality of channelcode groups includes at least (i) a first code group including a firstplurality of orthogonal channel codes of the set of orthogonal channelcodes, (ii) a second code group including a second plurality of channelcodes of the set of orthogonal channel codes, and (iii) a third codegroup including a third plurality of channel codes of the set oforthogonal channel codes, wherein the channel codes of the first codegroup are distinct from the channel codes of the second code group andthe third code group, and wherein the channel codes of the second codegroup are distinct from the channel codes of the third code group;assigning the first code group to the first beam zone and to the secondbeam zone such that the channel codes of the first code group areuseable within the first beam zone and the second beam zone, assigningthe second code group to the third beam zone such that the channel codesof the second code group are useable within the third beam zone, andassigning the third code group to the fourth beam zone such that thechannel codes of the third code group are useable within the fourth beamzone, and while a given channel code of the first code group is in useby a first mobile station within the first beam zone, assigning thegiven channel code to a second mobile station within the second beamzone so that the given channel code is simultaneously in use by thefirst base station in the first beam zone and by the second mobilestation in the second beam zone.
 2. The method of claim 1, wherein thechannel codes are dynamically assigned to the first code group, thesecond code group, and the third code group for use within the multiplebeam zones.
 3. The method of claim 2 wherein the number of channel codesthat may be assigned for use in each beam zone of the multiple beamzones is subject to a maximum number of channel codes.
 4. The method ofclaim 3, wherein the maximum number of channel codes for each beam zoneis dynamically adjusted.
 5. The method of claim 4, wherein the maximumnumber of channel codes for each beam zone is adjusted in response tocall statistics of the sector.
 6. The method of claim 4, wherein themaximum number of channel codes for each beam zone is adjusted inresponse to time of day.
 7. The method of claim 1, wherein the channelcodes are statically assigned for use within the multiple beam zones. 8.The method of claim 7, wherein the first code group, the second codegroup, and third code group are periodically modified.
 9. The method ofclaim 8, wherein the periodic modification of the first code group, thesecond code group, and the third code group is based on statisticalusage patterns.
 10. The method of claim 1, wherein the first code group,the second code group, and the third code group each comprise adifferent number of channel codes.
 11. The method of claim 1, furthercomprising: prior to assigning the given channel code to a second mobilestation within the second beam zone, making a determination that alocation of the second mobile station is fixed.